In core drilling, a core tube is suspended inside a drill string for receiving a core sample of the ground being cut by the drill. The core tube is coupled to a lower end of an inner core barrel head assembly which is releasably locked inside the drill string. In order to retrieve a core sample received in a core tube, an overshot is lowered through the drill string at the end of a wire line to engage a spear point attached to an upper end of the inner core barrel head assembly. Similarly, once the core is extracted from the core tube, the inner core barrel head assembly is lowered into the drill string using the overshot and wire line.
Typically an inner core barrel head assembly is provided with a pair of spring loaded latch dogs that engage in an annular latch seat formed on the inside surface of the drill string. This seat is often formed as a increased inner diameter portion of an adaptor coupling used for coupling a lower most drill rod of a drill string to a core barrel outer tube. By pulling on the wire line, one part of the inner core barrel head assembly moves relative to another causing latch dogs to pivot inwardly toward each other and disengage from the latch seat. Once this occurs, further pulling on the wire line causes the head assembly to be pulled from the drill string. However, occasionally the latch dogs become jammed and are unable to be disengaged from the latch seat. In such instances, the only way the head assembly can be removed is to pull the drill string. This is an extremely costly exercise because of the time involved in breaking out the individual drill rods and subsequent re-assembly and the corresponding loss of drill time.
Several attempts have been made to overcome this problem such as in U.S. Pat. No. 6,089,335 (Able) and U.S. Pat. No. 5,934,393 (Marshall). In Able, the latch dogs are inverted with respect to the common configuration so that a main pivot point of the latch dogs is disposed upstream of their corresponding latching face. In Marshall, the latch dogs are in their traditional configuration but a mechanical link is provided between the free ends of the latch dogs to pivot them inwardly toward each other and thus away from the latch face by upward pulling on the wire line.